Curable epoxy resin compositions are used as sealants, adhesive agents, or similar substances for parts of electric and electronic devices. However, since cured products obtained by curing the compositions have a high modulus of elasticity and, therefore, high rigidity, such agents and sealants may increase stress in the aforementioned electric and electronic parts when subject to thermal expansion or curing contraction. It is known in the art to reduce the modulus of elasticity in a cured body of the composition by adding to the composition a silicone rubber powder that contains epoxy groups (see Japanese Unexamined Patent Application Publication S64-51467).
However, even with an addition of a silicone rubber powder the curable epoxy resin composition cannot provide sufficient decrease of modulus of elasticity and, in addition, impairs mechanical strength of the cured product. Based on a profound study conducted by the inventors herein for finding the factors that do not allow sufficient decrease of modulus of elasticity, it has been revealed that this occurs either because of a low content of epoxy groups in the silicone rubber powder and insufficient reaction thereof in the curable epoxy resin composition, or because the silicone rubber powder obtained by an addition reaction has dispersity in organic resins inferior to the silicone rubber powder obtained by a condensation reaction.
It is an object of the present invention to provide a curable epoxy resin composition that possesses excellent flowability and that, when cured, forms a cured product which, in spite of low modulus of elasticity, demonstrates high strength.
It is another object to provide a cured product demonstrates low modulus of elasticity in combination with high strength.